Field of the Invention
The present invention relates to a sheet stacking apparatus configured to sequentially stack delivered sheets, and an image forming apparatus comprising the sheet stacking apparatus.
Description of the Related Art
Hitherto, a sheet stacking apparatus includes a stack tray on which delivered sheets are sequentially stacked, a raising and lowering portion configured to raise and lower the stack tray, an upper-surface detection sensor configured to detect an upper surface of a topmost sheet of the sheets stacked on the stack tray, and a control portion configured to control the raising and lowering portion based on a result of detection so that the upper surface of the sheets stacked on the stack tray is controlled to be constantly positioned at a predetermined height level.
In this type of sheet stacking apparatus, however, when a large number of sheets are removed from the stack tray, a position of the topmost sheet on the stack tray is lowered. Therefore, when a subsequently delivered sheet is introduced to the stack tray, a distance over which the sheet falls increases. As a result, there is a fear of failure in sheet delivery and failure in sheet stacking.
Thus, there has been known a sheet stacking apparatus further including a second sensor configured to detect removal of a part of a bundle of sheets from a stack tray. The sheet stacking apparatus moves the stack tray by a raising and lowering portion based on the detection result of the second sensor so that the stack tray returns to an appropriate sheet delivery position (Japanese Patent Application Laid-Open No. H11-199114).
FIG. 8 is a front view for illustrating a first sensor 100, a second sensor 200, and a stack tray 400 in a related-art sheet stacking apparatus. The first sensor 100 is a transmission sensor including a light-receiving portion 100a, and the second sensor 200 is a transmission sensor including and a light-receiving portion 200a. The first sensor 100 and the second sensor 200 share a light-emitting portion 300.
The first sensor 100 forms a first optical axis L1 between the light-receiving portion 100a and the light-emitting portion 300 respectively mounted to an upper part of a left side and an upper part of a right side of the stack tray 400. The light-receiving portion 100a and the light-emitting portion 300 are arranged so that the optical axis L1 becomes parallel to a rear edge of a bundle of sheets S in a state of being well-stacked on the stack tray 400.
The second sensor 200 forms a second optical axis L2 between the light-receiving portion 200a and the light-emitting portion 300. The light-receiving portion 200a of the second sensor 200 is arranged below the light-receiving portion 100a of the first sensor 100. Therefore, the optical axis L2 of the second sensor 200 is set at an angle with respect to the horizontal optical axis L1 of the first sensor 100.
The first sensor 100 is used to lower the stack tray 400 until the optical axis L1 is restored after the optical axis L1 is interrupted by a bundle of sheets S stacked on the stack tray 400. On the other hand, the second sensor 200 is used to raise the stack tray 400 until the optical axis L2 is interrupted again after the bundle of sheets S on the stack tray 400 is partially or entirely removed to open the interrupted optical axis L2.
With the sensor configuration described above, however, when the sheet introduced to the stack tray 400 has a curled edge or has a partially swelled edge through a binding process, a surface of the sheet is not level. Therefore, the sheet cannot be detected at an appropriate timing. As a result, there arises a fault that raising and lowering control of the stack tray 400 is adversely affected.
More specifically, the above-mentioned fault will be described referring to FIG. 9A to FIG. 9F. FIG. 9A to FIG. 9F are explanatory diagrams of the fault occurring during the raising and lowering operation of the stack tray according to the related art. FIG. 9B, FIG. 9D, and FIG. 9F are side views of the stack tray 400 on which the sheets delivered by delivery rollers 71 in a direction indicated by an arrow A are stacked. FIG. 9A, FIG. 9C, and FIG. 9E are front views of the stack tray 400 as viewed from a direction opposite to the direction indicated by the arrow A of FIG. 9B, FIG. 9D, and FIG. 9F, respectively.
In FIG. 9A and FIG. 9B, a height level of an upper surface of the bundle of sheets S stacked on the stack tray 400 is positioned between the light-receiving portion 100a and the light-receiving portion 200a. Therefore, the bundle of sheets S is positioned sufficiently below the optical axis L1. Hence, the optical axis L1 is not interrupted even when a subsequent sheet is stacked thereon, and therefore the stack tray 400 is not lowered. Further, the optical axis L2 is interrupted by the bundle of sheets S. Unless the optical axis L2 is opened by removing the bundle of sheets S entirely or partially, the stack tray 400 is not raised.
When a sheet of which a surface is not level and swells along inclination of the optical axis L2, for example, a sheet C having a curled rear edge is introduced in the above-mentioned sheet stacking state, the curled portion interrupts the optical axis L1. As a result, a lowering operation of the stack tray 400 is performed.
Then, when the curled portion of the sheet C deviates from the optical axis L1, a drive to lower the stack tray 400 is stopped. However, the lowering is continued by inertia for a while (FIG. 9C and FIG. 9D).
At this time, the sheet C has the curled portion to result in the uneven surface, and therefore has an approximately triangular large interruption region P that interrupts the optical axis L2. Therefore, when the lowering is perfectly completed, an upper surface of the sheet C reaches a position at which the optical axis L2 of the second sensor 200 is opened (FIG. 9E and FIG. 9F). Thus, the bundle of sheets S is regarded as having been removed from the stack tray 400. As a result, a raising operation of the stack tray 400 is performed.
When the optical axis L2 is interrupted again by the curled portion through the raising of the stack tray 400, the drive to raise the stack tray 400 is stopped (FIG. 9C and FIG. 9D). Even at this time, the raising is continued by inertia for a while, and the curled portion interrupts the optical axis L1 again (FIG. 9A and FIG. 9B). Then, the lowering operation of the stack tray 400 is restarted. Subsequently, there is brought about a loop operation in which the lowering and the raising of the stack tray 400 described above are repeated, resulting in an erroneous operation that a topmost sheet stacked on the stack tray 400 as the stack portion cannot be positioned at a predetermined position.